mutex.rs: remove needless-maybe-unsized bounds

[hkBst]

Fixes for:

warning: `?Sized` bound is ignored because of a `Sized` requirement
   --> library/std/src/sync/nonpoison/mutex.rs:425:9
    |
425 | impl<T: ?Sized + Default> Default for Mutex<T> {
    |         ^^^^^^
    |
note: `T` cannot be unsized because of the bound
   --> library/std/src/sync/nonpoison/mutex.rs:425:18
    |
425 | impl<T: ?Sized + Default> Default for Mutex<T> {
    |                  ^^^^^^^
    = note: ...because `Default` has the bound `Sized`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_maybe_sized
    = note: `-W clippy::needless-maybe-sized` implied by `-W clippy::suspicious`
    = help: to override `-W clippy::suspicious` add `#[allow(clippy::needless_maybe_sized)]`
help: change the bounds that require `Sized`, or remove the `?Sized` bound
    |
425 - impl<T: ?Sized + Default> Default for Mutex<T> {
425 + impl<T: Default> Default for Mutex<T> {
    |

warning: `?Sized` bound is ignored because of a `Sized` requirement
   --> library/std/src/sync/poison/mutex.rs:691:9
    |
691 | impl<T: ?Sized + Default> Default for Mutex<T> {
    |         ^^^^^^
    |
note: `T` cannot be unsized because of the bound
   --> library/std/src/sync/poison/mutex.rs:691:18
    |
691 | impl<T: ?Sized + Default> Default for Mutex<T> {
    |                  ^^^^^^^
    = note: ...because `Default` has the bound `Sized`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_maybe_sized
help: change the bounds that require `Sized`, or remove the `?Sized` bound
    |
691 - impl<T: ?Sized + Default> Default for Mutex<T> {
691 + impl<T: Default> Default for Mutex<T> {

[rustbot]

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[jhpratt]

@bors r+ rollup

[bors]

📌 Commit b2e6e03 has been approved by jhpratt

It is now in the queue for this repository.